The main objective of the present study is to elucidate the relationship between the gonadal steroid environment and the organization and function of a steroid-sensitive cholecystokinin (CCK) circuit which is part of a sexually dimorphic limbic- hypothalamic system. This interconnected system, including the medial amygdaloid nucleus, bed nucleus of the atria terminalis, medial preoptic nucleus and the ventromedial region of the hypothalamus, is implicated in the integration of sensory and sex steroid cues involved in the CNS control of reproductive behavior. The sexually dimorphic distribution of the neuroactive peptide CCK in the limbic-hypothalamic system appears to be an important chemical marker for the action of steroids, especially since in this system the mechanism of steroid-regulation of CCK synthesis in males may be different from that in females. Initially, we propose to characterize the interaction between gonadal steroids and the CCK circuit in adult males by determining the metabolite of testosterone necessary to maintain the limbic-hypothalamic CCK circuit. To examine the steroidal influence on CCK release, in vitro superfusion will be utilized. In order to obtain a morphological signature of steroidal action on CCK cells, a combination of steroid autoradiography and CCK- immunohistochemistry will be used to determine if CCK cells in the male concentrate steroids. To begin to elucidate the integration of steroid information within the limbic-hypothalamic system, retrograde tract-tracing will be combined with steroid autoradiography and immunohistochemistry. The role of CCK in the male limbic-hypothalamic system will be tested by injecting CCK into discrete loci in this system. Finally, to understand the organizing-role of gonadal steroids during development, a series of rats will have their perinatal sex-steroid environment altered to produce sex-reversed adults in terms of their reproductive behavior. In two parallel groups of rats, the distribution of CCK- positive cells in the limbic-hypothalamic system of these rats will be determined and correlated with the effects of CCK on reproductive behavior. Together these studies will assess the role of CCK in male copulatory behavior and the effects steroids have on modulating the distribution and release of CCK in male rats. These proposed studies are expected to shed significant light on the important questions of how steroidal information is encoded and integrated in the CNS to influence reproductive behavior.